1. Field of the Invention
The present invention relates to a method of manufacturing a photomask. More particularly, the present invention relates to a method of manufacturing a rim type of photomask.
2. Description of the Related Art
In general, an exposure apparatus for use in forming a circuit pattern on a semiconductor substrate includes a mask or reticle having a mask pattern corresponding to the circuit pattern that is to be formed, an exposure light source for illuminating the mask or reticle with an exposure light so that an image of the pattern of the mask or reticle is picked up by the exposure light, and a pupil lens for condensing the exposure light onto a photoresist on the semiconductor substrate. In this way, an image of the pattern of the mask or reticle is transferred to the photoresist on a reduced scale. Then, the photoresist is developed to selectively remove the exposed or unexposed portions thereof such that a photoresist pattern is formed on the semiconductor substrate.
One known type of mask or reticle is a binary mask. An ordinary binary mask consists of a quartz substrate and an opaque pattern(light blocking layer), for ample, a chrome pattern, disposed on the quartz substrate. However, if a line width of the opaque pattern is smaller than the wavelength (λ) of the exposure light, the angle of diffraction of the exposure light passing through the mask is too great for the exposure light to be focused on the pupil lens of the exposure apparatus, and the image being transferred by the diffracted light will not have a sufficient amount of contrast. Therefore, it is problematic to use a binary mask having an opaque pattern to form a fine circuit pattern, i.e., a circuit pattern having a small line width.
In light of this, phase shift masks using destructive interference have been developed. The phase shift masks are classified as follows: attenuated phase shift masks having a 180°-phase shift region formed of a phase shift material, for example, MoSiON, which transmits a small percentage of the exposure light; alternating phase shift masks having a 180°-phase shift region and a chrome layer formed in a trench in the quartz substrate; chromeless phase shift masks having 0°- and 180°-phase shift regions formed at different depths in the quartz substrate, e.g., and in which the 0°-phase shift region is constituted by the surface of the quartz substrate whereas the 180°-phase shift region is constituted by a trench in the substrate; and rim type of photomasks (hereinafter, referred to as rim masks) in which a chrome pattern is disposed on a 0°-phase shift region of what would otherwise be a chromeless phase shift mask.
FIG. 1 is a graph illustrating the contrasts of the images transferred by various types of photomasks. Referring to FIG. 1, the rim mask can provide a higher degree of contrast than the binary mask and the attenuated phase shift mask. That is, the rim mask provides the highest and lowest intensities in the light transmitted therethrough, and thus can be used to form the finest photoresist pattern among the three masks.
FIG. 2 is a top view of a conventional rim mask. Referring to FIG. 2, the rim mask includes a quartz substrate 10 having 0°-phase shift and 180°-phase shift regions 10a and 10b. The 0°-phase shift region 10a is defined by the surface of the quartz substrate 10 and the 180°-phase shift region 10b is defined by a trench having a predetermined depth in the quartz substrate 10. In addition, the rim mask includes a chrome pattern 15 on the 0°-phase shift region 10a. Also, the chrome pattern 15 leaves a border 10c of the 0°-phase shift region 10a exposed adjacent the 180°-phase shift region 10b. The rim mask is manufactured as shown in FIGS. 3A through 3C.
Referring first to FIG. 3A, a chrome layer is formed on the quartz substrate 10. A first photoresist pattern 14 is formed on the chrome layer using electron beam lithography. The chrome layer is etched to attain the shape of the first photoresist pattern 14. In addition, the quartz substrate 10 is etched to a predetermined depth using the first photoresist pattern 14 and the etched chrome layer 12 as a mask to define the 180°-phase shift region 10b. Accordingly, the 0°-phase shift region 10a is defined as well.
Referring to FIG. 3B, the first photoresist pattern 14 is removed. Next, a photoresist layer is formed on the resultant structure and is etched using electron beam lithography to form a second photoresist pattern 16 on the etched chrome layer 12. The second photoresist pattern 16 exposes an outer peripheral portion of the etched chrome layer 12, i.e., corresponding to the border 10c of the 0°-phase shift region 10a. 
Referring to FIG. 3C, the chrome layer 12 is etched once more using the second photoresist pattern 16 as a mask to form a chrome pattern 15 that exposes the quartz substrate 100 at the border 10c. Then, the second photoresist pattern 16 is removed.
However, the following problems arise when manufacturing the rim mask.
First, the second photoresist pattern 16 is theoretically designed to define the border 10c. However, it is very difficult to align the second photoresist pattern 16 on the etched chrome layer 12 such that a peripheral region of the etched chrome layer 12, corresponding precisely to the border 10c, is left exposed by the second photoresist pattern 16. In addition, expensive equipment such as electron beam and/or laser equipment is required for rectifying any misalignment which may occur between the second photoresist pattern 16 and the etched chrome layer 12.
Second, the first and second photoresist patterns 14 and 16 are charged with electrons because the first and second layers of photoresist, from which the photoresist patterns 14 and 16 are formed, are exposed using electron beams. The charged electrons are scattered non-uniformly throughout the layers of photoresist when the layers are developed, i.e., patterned. As a result, the first and second photoresist patterns 14 and 16 are irregular. Accordingly, the chrome pattern 15 also is irregular.
A process of exposing the photoresist patterns using a short wavelength laser has been suggested as a way to prevent the photoresist layers from being charged. However, such a laser provides inferior exposure in a region that has a dimension on the order of microns, such as the region corresponding to the border 10c. A process of employing a conductive polymer in order to discharge the electrons has also been suggested, but is expensive and complicated.